The invention relates to a polymerization process for olefinically unsaturated monomers utilizing alkyl-substituent containing metallocenes to achieve polymer products having narrow composition distributions, low melt index ratios, and, optionally, high melt strength.
The use of metallocene compounds in catalyst systems for the polymerization of olefinically unsaturated olefins is well known in the art. Metallocene compounds have been defined as organometallic coordination compounds obtained as a cyclopentadienyl derivative of a transition metal or metal halide. Three types are representative: biscyclopentadienyl Group 4 metal compounds, biscyclopentadienyl Group 5-10 metal compounds and monocyclopentadienyl Group 4 and 5 metal compounds. In these categories are included those having alkyl-substituents on one or both cyclopentadienyl rings, both bridged metallocenes and unbridged metallocenes where the bridging if present is between one cyclopentadienyl ring ligand and another, or one cyclopentadienyl ring ligand and a heteroatom ligand of the transition metal. Syntheses of these compounds are well known and typically comprise the reaction of one or more selected cyclopentadienide compound with a transition metal halide.
Metallocenes generally are capable of narrow molecular weight distributions (MWD) in view of their typically yielding single-site catalysts when activated. However, capabilities as to compositional distributions (CD) and melt index ratios (MIR) vary widely, particularly when the transition metal compounds are immobilized on particulate supports so as to form heterogeneous catalyst systems. Polymer products used in certain applications benefit from low MIR. The optical properties are improved when polymer fractions, typically comprised in the polymerization reaction product of any coordination polymerization catalysts, are as similar as possible. Additionally, narrow CD polymers typically have good optical properties, low levels of extractables and low blocking attributes. The narrow CD also is indicative of narrow melting point distribution which is of particular importance for film manufacturers.
Unbridged biscyclopentadienyl metallocene compounds having alkyl substituents said to be suitable for olefin polymerization, particularly heterogeneous polymerization process conditions, are described in U.S. Pat. No. 5,470,811. The catalysts comprise preferably at least two alkyl substituents on each and provide broad molecular weight distribution, high molecular weight and narrow composition distributions. Table II examples 11-13 illustrate the use of (1,3-Me-n-BuCp)2ZrCl2, (1,2-Me-n-BuCp)2ZrCl2 and (n-Pr-Cp)2ZrCl2 for copolymers having I21/I2 (defined as melt index ratio, MIR, measured in accordance with ASTM D-1238) values from 17.9 to 23.2 and MWD (Mw/Mn) values of from 2.5 to 3.7. Utility of the copolymers in film products and background for film preparation are disclosed. See also, WO 96/00246 for a description of multiply substituted cyclopentadienyl ring-containing metallocenes and their use in gas phase polymerization processes where low melt indices or high molecular weights are sought.
Bridged biscyclopentadienyl metallocenes useful for gas phase polymerization are disclosed in European Patent Application 0 659 773 A1. The cyclopentadienyl moieties may be substituted with one or more substituents R, the catalysts are said to be suitable for the production of long chain branch-containing polyethylene when used in the process described. Values for I21/I2 of 56 to 110 are illustrative of the invention, the comparative examples illustrate values of 37 to 47.
The invention comprises an olefin polymerization process comprising contacting ethylene alone or with one or more olefinically unsaturated comonomers with a Group 3-6 metallocene catalyst compound comprising one xcfx80-bonded ring having one more C3 or greater hydrocarbyl, hydrocarbylsilyl or hydrocarbylgermyl substituent, said substituent bonded to the ring through a primary carbon atom; and, 2) where a metallocene containing two xcfx80-bonded rings, the total number of substituents on the rings is equal to a number from 3 to 10, said rings being asymmetrically substituted where the number of substituents is 3 or 4. In a preferred embodiment compound containing two xcfx80-bonded rings, the rings are covalently bridged and a C3 or greater hydrocarbyl, hydrocarbylsilyl or hydrocarbylgermyl substituent is at its ring 3 or 4 position with respect to the bridge. Where the metallocene contains only one xcfx80-bonded ring, it will preferably be bridged to a Group 15 or 16 heteroatom, said heteroatom preferably containing a 2xc2x0 or 3xc2x0 Group 14 atom containing hydrocarbyl, hydrocarbylsilyl or hydrocarbylgernyl substituent group of 1 to 20 non-hydrogen atoms. The term substituent when referring to those groups substituted on the rings includes essentially hydrocarbyl, hydrocarbylsilyl or hydrocarbylgermyl groups having from about 1 to 30 Group 14 atoms. Two adjacent ring substituent groups may be joined so as to form a fused ring system, for example, unsubstituted or further substituted indenyl, fluorenyl or azulenyl groups. The invention process is particularly suitable for preparing ethylene copolymers having an MIR less than about 35, preferably less than about 30, and for certain embodiments less than about 20, and narrow CD, such as those having a comonomer distribution breadth index (CDBI) equal to or above 60, even at high comonomer incorporation rates. Ethylene copolymers having improved melt strength while retaining the low MIR and narrow CD are also enabled by use of invention metallocene compounds containing two xcfx80-bonded rings covalently bridged to each other, with the C3 or greater hydrocarbyl, hydrocarbylsilyl or hydrocarbylgermyl substituent being bonded at the 3 or 4 position on one of the rings, where the ring carbon covalently bonded to the bridge is counted as the 1 position.